Section steel and method for manufacturing same

ABSTRACT

A section steel according to an exemplary embodiment of the present invention is characterized in that it includes an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other unavoidable impurities, and has a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

TECHNICAL FIELD

Exemplary embodiments of the present invention relate to section steel and a method for manufacturing the same, and more particularly, to high-strength and high-performance section steel having fireproof/aseismic performance and a method for manufacturing the same.

BACKGROUND

A section steel generally refers to a steel material whose cross-sectional shape is variously changed. Recently, the section steel has been applied as a structural steel material such as pillars of large buildings, and has also been applied as temporary materials for civil works such as subways and bridges and piles for foundation. The section steel may be manufactured by hot rolling a cast piece such as bloom, billet, beam blank, and the like. manufactured by continuous casting.

Recently, large-scale earthquakes have occurred around the world, resulting in enormous loss of life and property. Even in Korea, strong earthquakes with a magnitude of 5.0 or higher occurred one after another in Gyeongju and Pohang in 2016 and 2017, which increased anxiety. When an earthquake occurs, a fire that may occur secondarily together with the primary damage caused by building damage causes softening of a reinforcement supporting the structure, which may accelerate building collapse along with plastic deformation of the reinforcement due to the earthquake. Accordingly, building design standards have recently been strengthened to minimize damage to life and property by delaying the collapse of buildings even in disaster situations such as earthquakes or high-rise building fires. In order to strengthen the safety of such buildings, it is essentially required to improve aseismic and fireproof performance of materials for building structures used in structure fabrication along with the aseismic design of the buildings, the installation of protective facilities such as sprinklers, etc. For this purpose, aseismic steel with secured aseismic performance capable of withstanding earthquakes through yield ratio control and refractory steel capable of withstanding fire through improvement of high-temperature strength have been developed and used. However, since damage to the building may result in a fire when an earthquake occurs as mentioned above, there is an increasing demand for 490 MPa class fire- and earthquake-resistant section steel that has both aseismic performance and fireproof performance to prepare for these situations.

As a related prior art, there is Korean Patent Application Laid-Open No. 10-2014-0056765 (published on May 12, 2014, title of the invention: Section Steel and Method for Manufacturing Same).

SUMMARY OF THE INVENTION Technical Problem

An object of the present invention is to provide high-strength and high-performance section steel having fireproof/aseismic performance and a method for manufacturing the same.

Technical Solution

A section steel according to an exemplary embodiment of the present invention for achieving the above object is characterized in that it includes an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other unavoidable impurities, and has a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

The section steel may have a shock absorption energy of 200 J or greater at 0° C.

A final microstructure of the section steel may include bainite.

A method for manufacturing section steel according to an exemplary embodiment of the present invention for achieving the above object comprises the steps of: (a) reheating a steel material including an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or greater of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other unavoidable impurities to 1,200 to 1,250° C.; (b) hot-rolling the steel material so that a rolling end temperature becomes 910 to 950° C.; and (c) subjecting the hot-rolled steel material to QST (Quenching & Self-Tempering) treatment.

In the method for manufacturing section steel, the QST (Quenching & Self-Tempering) treatment step may have a water-cooling end temperature and a self-tempering temperature of 765 to 800° C.

In the method for manufacturing section steel, the section steel which has been subjected to the step (c) may have a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

The step (b) of the method for manufacturing section steel may comprise a step of hot-rolling the steel material so that a rolling start temperature becomes 1,050 to 1,100° C.

Advantageous Effects

According to embodiments of the present invention, it is possible to implement high-strength and high-performance section steel having fireproof/aseismic performance and a method for manufacturing the same. Of course, the scope of the present invention is not limited by these effects.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a flowchart illustrating a method for manufacturing section steel according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, a section steel according to an exemplary embodiment of the present invention and a method for manufacturing the same will be described in detail. The terms described below are terms appropriately selected in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout the present specification.

In the recent trend of high-rise building structures, it is essential to design safe structures in preparation for disasters such as fire or earthquake, and the development of high-functional construction materials such as fire resistance and earthquake resistance is urgently needed. Meanwhile, the demand for safety design for securing disaster safety of buildings in case of fire is also being strengthened. Europe including the UK, the United States, Australia, etc. has raised the level of requirements for safety design through the revision of laws and regulations for fire-resistance design of ultrahigh-rise buildings. Japan, which has a similar regulatory system for building laws to that of Korea, have amended the building standards law and have applied performance regulations for fire resistant structures and regulations on fireproof performance. Although fireproof thick plate materials have been developed in Korea, they are not commercially available, and there is no development and performance evaluation of fire-resistant steel materials for steel materials (H-steel, etc.) for building structures with shapes. Hereinafter, high-strength and high-performance section steel having stable fireproof/aseismic performance and a method for manufacturing the same will be described.

Section Steel

Section steel according to an exemplary embodiment of the present invention includes an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component contained in the section steel according to an exemplary embodiment of the present invention will be described.

Carbon (C)

Carbon (C) is an element that is added to secure strength and has the greatest influence on weldability. Further, carbon reacts with Nb, Ti, etc. to promote the formation of fine carbides, thereby effectively contributing to strength improvement through precipitation strengthening, and also impedes dislocation movement at high temperatures to improve high-temperature strength, which makes it possible to effectively secure fireproof performance. The carbon (C) may be added at a content ratio of 0.08 to 0.17% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of carbon is less than 0.08% by weight of the total weight, it may be difficult to secure sufficient strength. When the content of carbon exceeds 0.17% by weight of the total weight, problems may arise in that coarse carbides are formed to not only reduce the impact properties but also generate discontinuous yield behavior, thereby increasing the yield ratio so that aseismic performance is reduced, lowering impact toughness of a base material, and lowering weldability during electric resistance welding (ERW).

Manganese (Mn)

Manganese (Mn), as a solid solution strengthening element, is an element which is effective in generating bainite structure by not only contributing to securing strength, but also improving the hardenability of steel. Manganese may be added at a content ratio of 0.50 to 1.60% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of manganese is less than 0.50% by weight, the effect of solid solution strengthening cannot be sufficiently exhibited. Further, when the content of manganese exceeds 1.60% by weight, it may combine with S to form MnS inclusions or may cause central segregation to be generated in the ingot, thereby lowering ductility of the section steel and lowering the corrosion resistance.

Silicon (Si)

Silicon (Si) is added together with aluminum as a deoxidizer for removing oxygen in steel in the steelmaking process. Further, silicon may also have a solid solution strengthening effect. The silicon may be added at a content ratio of 0.10 to 0.50% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of silicon is less than 0.10% by weight of the total weight, the effect of adding silicon may not be properly exhibited. When the content of silicon is added in a large amount exceeding 0.50% by weight of the total weight, the weldability of steel may be lowered, a red scale may be formed during reheating and hot rolling, which causes a problem with the surface quality.

Chromium (Cr)

Chromium (Cr) is an element that contributes to securing bainite microstructure by improving the hardenability of steel, and when chromium (Cr) is added to C—Mn steel as a ferrite stabilizing element, it delays the diffusion of carbon due to the solute interference effects, thereby affecting particle size refinement. The chromium may be added at a content ratio of 0.10 to 0.70% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of chromium is less than 0.10% by weight of the total weight, the effect of adding chromium may not be properly exhibited. When the content of chromium is added in a large amount exceeding 0.70% by weight of the total weight, problems may arise in that the manufacturing cost of steel is increased, coarse carbides are formed at grain boundaries so that the ductility of the steel is lowered, and the properties of the steel are lowered from the viewpoint of toughness and hardenability.

Copper (Cu)

Copper (Cu) is an element that is solid-solutioned in ferrite to exhibit a solid solution strengthening effect. Further, in the bainite transformation, copper that is not precipitated and is supersaturated is solid-solutioned in the structure at room temperature, a copper phase is precipitated on dislocations introduced by bainite transformation when it is heated to a use temperature of refractory steel of 600° C., and the strength of the base material is increased by the precipitation hardening. The copper may be added at a content ratio of greater than 0 and 0.5% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of copper is added in a large amount exceeding 0.5% by weight of the total weight, problems may arise in that hot working is difficult, precipitation strengthening is saturated, toughness is lowered, and red shortness occurs.

Molybdenum (Mo)

Molybdenum (Mo) is an element that may contribute to securing the bainite microstructure by improving the hardenability of steel and is very effective in securing high-temperature strength, and is an element that is effective in securing the strength and high-temperature strength of the base material. The molybdenum may be added at a content ratio of 0.30 to 0.70% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of molybdenum is less than 0.30% by weight of the total weight, the above-described effects may not be realized, and when the content of molybdenum is added in a large amount exceeding 0.70% by weight of the total weight, problems may arise in that the manufacturing cost of the steel is increased, the formation of grain boundary carbides is promoted so that the ductility of the steel is lowered, and the toughness of the base material and the weld heat affected zone deteriorates due to an excessive increase in quenching property.

Phosphorus (P)

Phosphorus (P) may allow solid solution strengthening to serve to increase the intensity of the strength and suppressing the formation of carbides. The phosphorus may be added at a content ratio of greater than 0 and 0.020% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of phosphorus exceeds 0.020% by weight, problems may arise in that inclusions, etc. are formed as tramp elements so that the ductility of the steel is reduced, and the low-temperature impact value is lowered by the precipitation behavior.

Sulfur (S)

Sulfur (S) may improve processability by forming fine MnS precipitates. The sulfur may be added at a content ratio of greater than 0 and 0.01% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of sulfur exceeds 0.01% by weight, inclusions, etc. may be formed as tramp elements so that the ductility of the steel is reduced, toughness and weldability may be impaired, and the low-temperature impact value may be lowered.

Nitrogen (N)

Nitrogen (N) may contribute to crystal grain refinement and contribute to securing high-temperature strength by forming nitride-based precipitates such as AlN, etc. The nitrogen may be added at a content ratio of greater than 0 and 0.012% by weight or less of the total weight of the section steel according to an exemplary embodiment of the present invention. When the content of nitrogen exceeds 0.012% by weight, the toughness of the weld may be lowered, and the impact value may be lowered.

Boron (B)

Boron (B) contributes to improving the strength of steel as a strong hardenable element. In the section steel according to an exemplary embodiment of the present invention, boron may be optionally added in an amount greater than 0 and 0.003% by weight or less. If the content of boron exceeds 0.003% by weight of the total weight of the section steel according to an exemplary embodiment of the present invention, a problem may arise in that material deviation is generated due to grain boundary segregation.

Nickel (Ni), Vanadium (V), Niobium (Nb), and Titanium (Ti)

Nickel (Ni) is an element that increases hardenability and improves toughness, vanadium (V) is an element that has an effect of increasing strength by forming precipitates during rolling, and in particular, is capable of controlling the precipitation amount depending on the amount of nitrogen added, niobium (Nb) is an element that is precipitated in the form of NbC or Nb(C, N) to improve strength of the base material and the weld, and titanium (Ti) is an element that suppresses the formation of AlN by high-temperature TiN formation and has the effect of refining the size of crystal grains by forming Ti(C, N), etc. The section steel according to an exemplary embodiment of the present invention contains at least one of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), but they may be added so that the sum of the contents thereof is 0.01 to 0.5% by weight of the total weight of the section steel. When the sum of the contents of at least one of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti) contained in the section steel according to an exemplary embodiment of the present invention is lower than 0.01% by weight, the above-described addition effects cannot be expected, and when it is higher than 0.5% by weight, problems may arise in that the manufacturing cost of parts is increased, brittle cracks occurs, and the carbon content in the matrix decreases so that the properties of steel are lowered.

The section steel according to an exemplary embodiment of the present invention having the above-described alloy element composition may have a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C. Further, it may have a shock absorption energy of 200 J or greater at temperature of 0° C.

Further, in the section steel according to an exemplary embodiment of the present invention having the above-described alloy element composition, the final microstructure may include bainite.

Hereinafter, a method for manufacturing the above-described section steel according to an exemplary embodiment of the present invention having the alloy element composition will be described.

Method for Manufacturing Section Steel

FIG. 1 is a flowchart schematically illustrating a method for manufacturing section steel according to an exemplary embodiment of the present invention. Referring to FIG. 1, a method for manufacturing section steel having excellent fire resistance properties according to an exemplary embodiment of the present invention comprises a reheating step (S100), a hot rolling step (S200), and a Quenching & Self-Tempering (QST) step (S300). The section steel rolling process is performed through a reheating process, a hot deformation process, and a cooling process. In the reheating process, a beam blank in a semi-finished product state is reheated to a temperature of 1,200 to 1,250° C. Next, the hot rolling process is characterized in that, after completing final finishing rolling at a temperature of 910 to 950° C. while passing the reheated beam blank through each of the rolling rolls (RM, IM, and FM), STT (Self Tempering Temperature) of 765 to 800° C. is secured through QST (Quenching and Self Tempering) equipment which is a surface accelerated cooling system.

First, in the reheating step (S100), a steel material of the predetermined composition described above is reheated. The steel material may be manufactured through a continuous casting process after obtaining molten steel of a desired composition through the steelmaking process. The steel material may be, for example, a billet or a beam blank.

The steel material may include an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other unavoidable impurities.

In an exemplary embodiment, the steel material may be reheated at a temperature of 1,200 to 1,250° C. When the steel material is reheated at the above-described temperature, components segregated during the continuous casting process may be re-solid solutioned. When a reheating temperature is lower than 1,200° C., problems may arise in that the solid solution of various carbides are not sufficient, and the components segregated during the continuous casting process are not sufficiently evenly distributed. When the reheating temperature exceeds 1,250° C., very coarse austenite grains are formed so that it may be difficult to secure strength. Further, when it exceeds 1,250° C., heating cost increases and process time is added, which may result in an increase in manufacturing cost and a decrease in productivity.

In the hot rolling step (S200), the reheated steel material is hot-rolled. The hot rolling may be controlled so that the rolling end temperature becomes 910 to 950° C. When the rolling end temperature is lower than 910° C., rolling is carried out in the non-recrystallization region so that the rolling addition may increase, and the yield ratio of the section steel as a rolling result may increase. Further, if the rolling end temperature exceeds 950° C., it may be difficult to secure the target strength and toughness. Meanwhile, the hot rolling may be controlled so that the rolling start temperature becomes 1,050 to 1,100° C.

In the QST (Quenching & Self-Tempering) step (S300), the hot-rolled section steel is cooled and self-tempered. A quenching method of spraying cooling water to the section steel is applied to the cooling. Further, the QST step may be performed in a state where the water cooling end temperature and the self-tempering temperature are controlled to 765 to 800° C. by controlling the feed rate of the section steel or the amount of cooling water sprayed.

Summarizing the above-described method for manufacturing a steel material, the steel material is manufactured through a reheating process, a hot deformation process, and a cooling process. In the reheating process, a billet or a beam blank in a semi-finished product state is reheated at a temperature of 1,200 to 1,250° C. Next, after hot-rolling the reheated material and performing the final finishing rolling at a temperature of 910 to 950° C. to complete transformation, the QST (Quenching & Self-Tempering) treatment may be performed in a state where the water cooling end temperature and the self-tempering temperature are controlled to 765 to 800° C. That is, in order to manufacture a rolled plate, after first reheating an ingot at a temperature of 1,200 to 1,250° C., hot rolling was performed to manufacture an H-beam, and at this time, the finish rolling temperature was controlled in a range of 910 to 950° C. After performing hot rolling to a thickness of 15 mm based on the flange part of the H-beam, cooling was performed. Water cooling was performed after hot rolling, and at this time, water cooling was performed by changing the water cooling end temperature to 765 to 800° C.

In embodiments of the present invention, steel grade design and process conditions with added chromium (Cr) and some alloying elements are applied so that strength and toughness can be improved while niobium (Nb) or titanium (Ti), which is an expensive precipitation hardening alloying element commonly used, is not used or is used in a small amount. Further, low-temperature toughness may be secured through self-tempering temperature control during the cooling.

It is possible to manufacture the section steel according to an exemplary embodiment of the present invention through the above-described manufacturing method. The manufactured section steel may have a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C. Further, the final microstructure may include bainite in the section steel according to an exemplary embodiment of the present invention.

EMBODIMENTS OF THE INVENTION Experimental example

Hereinafter, a preferred experimental example is presented to help the understanding of the present invention. However, the following experimental example is merely for helping understanding of the present invention, and the present invention is not limited by the following experimental example.

Table 1 shows main alloy element compositions (unit: % by weight) of the present experimental example, and Table 2 shows process conditions for manufacturing specimens of the present experimental example and results of measuring mechanical properties of the specimens implemented accordingly. After manufacturing beam blanks having the compositions of Table 1 using an electric furnace, H-steel having a 15 mm-thick flange portion were manufactured through hot rolling.

TABLE 1 Classification C Si Mn P S Cu Cr Mo Ti Nb B N Composition 0.16 0.23 1.40 0.023 0.013 0.24 0.60 0.45 0.001 0.002 — 0.008 system 1 Composition 0.16 0.20 1.38 0.019 0.009 0.20 0.62 0.35 0.05 0.017 0.002 0.01 system 2

TABLE 2 High- Rolling conditions Room temperature temperature Rolling Rolling tensile test yield Reheating start end Recuperation Tensile Yield strength Composition temperature temperature temperature temperature strength strength Yield at 600° C. Classification ratio (° C.) (° C.) (° C.) (° C.) (MPa) (MPa) ratio (MPa) Comparative Composition 1,220 1,070 936 680 693 472 0.69 243~273 Example 1 system 1 Comparative Composition 936 715 651 441 0.68 225~268 Example 2 system 1 Comparative Composition 926 729 578 373 0.65 262~279 Example 3 system 2 Comparative Composition 927 744 588 383 0.65 263~291 Example 4 system 2 Comparative Composition 929 762 583 366 0.63 272~286 Example 5 system 2 Example 1 Composition 933 785 579 383 0.66 283~306 system 2 Comparative Composition 936 802 569 366 0.64 228~265 Example 6 system 2 Comparative Composition 939 867 545 347 0.64 182~206 Example 7 system 2

Referring to Table 1, components of the composition system 2 of the present invention satisfy a composition including an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe). On the contrary, components of the composition system 1 of the present invention do not satisfy a composition comprising greater than 0 and 0.02% by weight or less of phosphorus (P), greater than 0 and 0.01% by weight or less of sulfur (S), and greater than 0 and 0.003% by weight or less of boron (B).

Referring to Table 2, the specimen according to Example 1 of the present experimental example satisfies a composition including an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe), and the process conditions satisfy a reheating temperature range of 1,200 to 1,250° C., satisfy a rolling start temperature range of 1,050 to 1,100° C., satisfy a rolling end temperature range of 910 to 950° C., and satisfy a range of 765 to 800° C. of a recuperation temperature that is the self-tempering temperature in the QST (quenching & self-tempering) treatment. Example 1, which satisfies such composition and process conditions, satisfies all of the requirements of a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

The specimen according to Comparative Example 1 of the present experimental example does not satisfy composition ranges including an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), and an amount greater than 0 and 0.003% by weight or less of boron (B). The recuperation temperature that is a self-tempering temperature in the QST (Quenching & Self-Tempering) treatment does not satisfy the range of 765 to 800° C. Accordingly, Comparative Example 1 does not satisfy the range of 490 to 620 MPa of the tensile strength at room temperature, and does not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

The specimen according to Comparative Example 2 of the present experimental example does not satisfy the composition ranges including an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), and an amount greater than 0 and 0.003% by weight or less of boron (B). The recuperation temperature that is the self-tempering temperature in the QST (Quenching & Self-Tempering) treatment does not satisfy the range of 765 to 800° C. Accordingly, Comparative Example 2 does not satisfy the range of 490 to 620 MPa of the tensile strength at room temperature, and does not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

The specimens according to Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6 of the present experimental example do not satisfy the range of 765 to 800° C. of the recuperation temperature that is the self-tempering temperature in the QST (Quenching & Self-Tempering) treatment. Accordingly, the specimens do not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

The specimen according to Comparative Example 7 of the present experimental example does not satisfy the range of 765 to 800° C. of the recuperation temperature that is the self-tempering temperature in the QST (Quenching & Self-Tempering) treatment. Accordingly, the specimen does not satisfy the yield strength of 355 MPa or greater at room temperature, and does not satisfy the high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention unless they do not depart from the scope of the present invention. Accordingly, the right scope of the present invention should be judged by the claims described below. 

1. A section steel comprising an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and an amount of 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other unavoidable impurities, wherein the section steel has a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
 2. The section steel of claim 1, wherein the section steel has a shock absorption energy of 200 J or greater at a temperature of 0° C.
 3. The section steel of claim 1, wherein a final microstructure of the section steel comprises bainite.
 4. A method for manufacturing section steel, the method comprising the steps of: (a) reheating a steel material comprising an amount of 0.08 to 0.17% by weight of carbon (C), an amount of 0.50 to 1.60% by weight of manganese (Mn), an amount of 0.10 to 0.50% by weight of silicon (Si), an amount of 0.10 to 0.70% by weight of chromium (Cr), an amount greater than 0 and 0.5% by weight or less of copper (Cu), an amount of 0.30 to 0.70% by weight of molybdenum (Mo), an amount greater than 0 and 0.02% by weight or less of phosphorus (P), an amount greater than 0 and 0.01% by weight or less of sulfur (S), an amount greater than 0 and 0.012% by weight or less of nitrogen (N), an amount greater than 0 and 0.003% by weight or less of boron (B), an amount of 0.01 to 0.5% by weight of the sum of at least one or more of nickel (Ni), vanadium (V), niobium (Nb), and titanium (Ti), and the remainder of iron (Fe) and other unavoidable impurities to 1200 to 1250° C.; (b) hot-rolling the steel material so that a rolling end temperature becomes 910 to 950° C.; and (c) subjecting the hot-rolled steel material to QST (Quenching & Self-Tempering) treatment.
 5. The method of claim 4, wherein the QST (Quenching & Self-Tempering) treatment step has a water cooling end temperature and a self-tempering temperature of 765 to 800° C.
 6. The method of claim 4, wherein the section steel which has been subjected to the step (c) has a tensile strength of 490 to 620 MPa, a yield strength of 355 MPa or greater, and a yield ratio of 0.8 or less at room temperature, and a high-temperature yield strength of 273 MPa or greater at a temperature of 600° C.
 7. The method of claim 4, wherein the step (b) comprises a step of hot-rolling the steel material so that a rolling start temperature becomes 1050 to 1100° C. 